The present invention relates to rotary mills. More particularly, the invention relates to a rotary mill with a rotating housing having a conical portion.
Rotary mills, also known as ball mills, pebble mills, rod mills, or tumble mills, are well known in the art. A traditional rotary mill includes a horizontal rotating cylinder or housing that rotates about a central axis. The cylinder includes grinding media that is generally spherical, cylindrical, or another shape. In the case of milling with a liquid medium, solid target materials are placed along with a liquid medium into the cylinder for milling. In the case of dry milling (without a liquid medium), the solid target materials are placed in the cylinder to be ground by the grinding media. The cylinder is rotated, causing the grinding media to tumble along with the target material, with the grinding media abrading and impacting the solid target materials. Continued rotation of the cylinder produces a milled product in the form of particles. In the case of milling with a liquid medium, the particles are suspended in the liquid. In the case of dry milling, the particles settle within the cylinder and between the grinding media.
Upon completion of the milling process, the milled product is discharged from the cylinder. The mill includes an opening with a solid cover. One method of discharge is known as dumping, and includes opening a solid cover of the cylinder and the milled solids and grinding media are dumped from the mill together out of the opening. The milled solids can then be separated with an ancillary device such as a stationary grate, vibrating sifter, or the like.
Another method of discharge is dry discharge, where the milled solids are separated from the grinding media and removed. The solid cover then can be manually removed and replaced with a discharge grate, which will retain the grinding media but allow the milled product to pass through the opening. Because the grinding media restricts the flow of the solids, the mill continues to rotate, so that existing solids will create a void space after exiting the cylinder and further solids will then fill the void space for subsequent discharge. This continues until most of the solids are removed.
In the case of a liquid milled product, a method of wet discharge can be used, which includes the grate for retaining the grinding media. The cylinder can remain stationary if the liquid suspending the product is a low-viscosity fluid, the liquid can flow past the media due to gravity. If, however, the liquid is a non-Newtonian or a high-viscosity liquid, the cylinder can be rotated to discharge the milled product.
The rotary mill also includes a discharge housing that surrounds the rotating cylinder to define an annular space between the cylinder and the housing. The housing also includes a collection hopper at its bottom. When the milled product is discharged, as described above, the milled product will enter the annular space and fall into the hopper.
However, the above discharge process can result in dirty conditions, with milled product adhering to the inner surface of the housing as well as the outside of the cylinder. Retrieval of the milled product from the discharge housing can also result in milled product entering the surrounding area. These conditions can reduce the amount of milled product recovered, as well as lead to cross-contamination issues and cleaning problems. In the case of liquid milling, the operator must make and break a liquid piped connection to the discharge housing, exposing the milled product and potential solvent vapors to the surrounding area during this break in the connection.
Additionally, it is difficult to adequately remove all of the milled product in the above described discharge methods. Milled product will tend to adhere to the inner surface of the cylinder as well as to the surfaces of the grinding media. This can lead to cross-contamination issues, which results when one batch of product is not fully discharged before the next batch is loaded, when the next batch is a different milled product. The residual product from the previous milling procedure can therefore become mixed with the subsequent product. The residual product in the cylinder is generally referred to as the “heel.”
One method for avoiding cross-contamination is to use separate mills for separate milled products. But this solution is costly and requires multiple dedicated mills, and is not feasible for many users.
There are acceptable levels of cross-contamination for various milling jobs, but the level that is acceptable varies and can only be defined by the particular requirements of each project.
Ball mills are typically not well suited for low cross-contamination because the grinding media itself provides a large surface are for the finely divided solids to adhere to, and further because the mill housing is in the form of a horizontally arranged cylinder. The cylindrical shape of the horizontally arranged cylinder results in product that becomes “trapped” at the ends of the cylinder. Due to the cylindrical shape of the mill housing and the horizontal orientation of the cylinder, the only force acting on a milled particle within the cylinder (gravity) is normal to the inner surface of the horizontal cylinder. Thus, there are no forces acting on the particles to move them horizontally.
When discharging dry solids from a horizontal mill cylinder, as described above, the dry solids exit the cylinder via a grate located on the periphery of the cylinder. The grate is typically arranged in the middle of the cylinder. Once most of the dry solids have been discharged, the remaining solids in the cylinder move more and more slowly toward the discharge grate. At this point, the movement toward the grate can be described as “random walk” or “Gaussian random walk.” For all practical purposes, the last of the solids do not exit the mill. The amount of time necessary to actually discharge all of the solids can be referred to as the “long tail” in reference to the tail of an exponential curve of the amount of solids discharged over time, where most of the solids are discharged early in the process, and the remaining solids are discharged more and more slowly over time.
Over an infinite period of time, all of the solids would eventually be removed. However, that is not reasonable. And even an extended period of rotating the mill to discharge more solids has disadvantages. The energy of tumbling the grinding media remains relatively constant even when there is no product left to be milled. As soon as discharge begins, the grinding media becomes more and more exposed to other grinding media. With fewer solids present, because they are being discharged, the energy of the grinding media is imparted onto other grinding media because the solids are no longer between the grinding media. Thus, extended rotation to remove more solids results in accelerated wear on the grinding media. Accelerated wear on the grinding media therefore requires that the media be replaced earlier and more frequently. Moreover, the additional wear on the grinding media results in grinding media material mixing the milled product.
Accordingly, allowing the cylinder to be rotated for extended periods of time to increase the amount of discharge and reduce the “heel” results in significant media wear and additional cross-contamination issues. Similar issues exist with liquid milling procedures.
Thus, there is a need for a discharge system that can reliably deliver the milled product from the cylinder more quickly and more completely.